The standard sails of a conventional sailing yacht mostly consist of two sails attached to a mast located approximately midships. The triangular head sail ahead of the mast is raised or pre-hoisted along a forestay which is braced approximately from the bow to the mast tip. The clew as the free end of the leech and foot of the sail is hauled tightly by means of a jibsheet so that the sail can adopt the most favourable possible aerodynamic profiled shape to the incoming wind which produces the greatest possible drive component due to large-volume wind deflection.
The directional downdraft of this head sail is intended to energetically intensify the lee flow of the subsequently positioned mainsail in such a manner that the most extensive possible all-over wind deflection with a maximum angle of deflection can be achieved without the lee flow becoming separated from the mainsail and becoming detached to form turbulence, which means loss of propulsion.
The mainsail is pre-hoisted behind the mast by means of a usual mast guide track connected to its luff whilst the foot of the sail is held on the main boom which is mounted on the mast so that it can pivot horizontally on both sides of the mast and can be hauled tightly with a mainsheet on the boom head.
On account of the better state and efficiency, the mainsail is usually provided with a plurality of flexible and pre-tensioned sail battens which make it possible to form an aerodynamically favourable sail profile having sufficient stability along with the usual opening of the leech which affords the desired increase in the sail surface for the same mast length.
The characteristic configuration of the sail profile and its angle of attack is crucial for the specific propulsion power per m2 of sail area. A great deal of technical effort is devoted primarily to the respective profile depth and its monitoring at all heights up the sail, such as sewn-in sail profiles, 3D laminates as sails and devices for controlled bending of the mast in the direction of travel. This usually takes place extremely precisely in the upper part of the sail but in the near zone of the straight boom the propulsion-bringing flow profile is increasingly deflected in a perturbing manner and ultimately interrupted by the sail boom, which is very much in the way in the direction of approach, producing turbulence.
Neither the angle of attack in the luff zone of a well-positioned main sail without edge joins, that is approximately corresponding to the incoming wind direction, nor the outflow angle at the leech following the forward-driving flow deflection inside the sail are similarly directed to the angle of attack of the boom. Thus, appreciable power losses occur in the transition zones from the sail profiled as correctly as possible in the upper region to the straight connected boom.
This is even greater since in this region the otherwise usable downdraft of the head sail does not meet an ordered lee flow on the rear side of the main sail since no such flow can be formed in this transition zone as a result of the unsatisfied requirements.
When the main sail is attached to the boom head, e.g. with a freely tensioned, therefore profilable foot of the sail, there is thus an increased possibility for the formation of wake turbulence along the foot of the sail, caused by exchange of air from the luff-side positive pressure zone to the lee side negative pressure zone. The tow turbulence thus produced and the turbulence losses of the straight boom not adapted to the direction of flow are at the expense of the drive energy and should be prevented as far as possible.
If the sail has a reef incorporated, e.g. in strong wind, and the correctly profiled sail from the upper region has become bound to the straight boom, a transition zone is also formed, whose sail area affords reduced propulsion power and due to its obliquely upwardly pitched profile, accordingly deflects the air flow upwards, which additionally leads to an elevated sail pressure point and more heeling with loss of power.
The following documents are published as prior art:    D1: U.S. Pat. No. 5,460,902 A (HEINSOHN AND MANION) 18 Apr. 1995 (18.04.1995). This describes a main boom which can adjust its shape to the sail profile; it consists of several segments. The section through this boom is rectangular.    D2: SU 1 512 858 A1 (PIVKIN AND EMELYANOV) 7 Oct. 1989 (07.10.1989) This also discloses a sail main boom consisting of several segments which are connected to one another in such a manner that the main boom can simulate the profiled shape of the mainsail. The cross-section through the boom is circular (FIG. 3).    D3: FR 2 557 064 A1 (BOISSON) 28 Jun. 1985 (28.06.1985)
This discloses a main boom comprising several segments so that its shape can be adjusted to the shape of the profile form of the mainsail. Here no inverted T-shape is provided in the cross-section.    D4: AT 504 907 B1 (WALDHAUSER KURT ING.) 15 Sep. 2008 (15.09.2008) (earlier-priority application)
This describes the cohesion of the individual sail boom segments and the control of the deflection of the sail boom by means of high-strength cord on the tension side as well as the absorption of compressive forces by means of a hard metal rocker bearings.